The present invention concerns a capacitive device for measuring the quality and/or degradation of a fluid, particularly an oil. The invention concerns in particular a device of this type for measuring the quality and/or degradation of a food oil, such as frying oil, directly in the cooking apparatus.
It is well known that food oils deteriorate during cooking, particularly when they are repeatedly brought to high temperatures. Typically, in order to fry food, these oils are brought to temperatures of the order of 180° C. At such temperatures a multitude of chemical reactions occur, such as polymerisations, thermo-oxidisations etc, which significantly alter the quality of the oil. The quantity of certain products of these reactions must not exceed a threshold imposed by law, since beyond such thresholds the oil is considered unfit for consumption. It is thus important to be able to detect the threshold in a reliable way in order to replace the oil as soon as it is necessary to do so. For a long time determination of the threshold was left to the cooks, who, after a visual or olfactory inspection, determined whether the oil was still fit for consumption. Of course, such a method is purely subjective and is consequently unreliable.
Various devices have been proposed in the prior art until now to try to overcome this problem in order to be able to measure objectively the quality and/or degradation of food oils. Since the degradation of food oils results in particular from the thermo-oxidisation thereof and this reaction produces polar compounds, devices have been envisaged in which the degree of degradation of the oil has been correlated with the dielectric constant of the oil, by measuring the capacitance of a capacitor in which the oil to be monitored forms the dielectric.
Such a device is disclosed for example in U.S. Pat. No. 5,818,731. This document discloses a device for measuring the quality of food oils to be mounted in a cooking apparatus such as a deep fryer. This device simultaneously monitors variations in the capacitance and optical transmission of the oil within the cooking or frying temperature range. The capacitive measuring unit comprises two sets of parallel plates interlaced with each other to define a measuring capacitor. When the sets of plates are immersed in oil, the latter forms the dielectric of the measuring capacitor for said unit and the variation in capacitance is measured by means of a direct current oscillator bridge circuit. However, this device has several drawbacks. A first drawback lies in the fact that the spaces been the plates are small and that, when the plates are plunged into the oil, the latter does not flow easily between the plates because of capillary phenomena. A regular change of the oil present between the plates is not, therefore guaranteed, which can lead to erroneous oil degradation measurement results. Moreover, solid particles present in the oil can also get caught between the plates, which has a negative effect on the measured signal. It should also be noted that, given the small space between the plates, the configuration of the capacitor in parallel plates does not allow easy access to these spaces, which makes it complex to carry out maintenance operations on the device. Another drawback lies in the fact that the capacitor with plates is bulky and occupies a significant amount of space on the cooking apparatus. Moreover, the single measuring sensor formed by the capacitor is subject to temperature variations, which can lead to erroneous capacitance measurements, such that the device has to provide means for compensating for such errors. The solution proposed in this document consists in using a temperature sensor that provides an indication to a suitable processing circuit to take account of temperature variations measured by software means integrating data relating to the oil to be monitored. Consequently, if the quality of the oil changes or if new oil is used, the software means have to be updated which makes the device inflexible to use.